Liquid crystal displays are used in a variety of applications including television sets, computer monitors, and smartphones. A liquid crystal display (LCD) includes an array of liquid crystal pixels, each having adjustable transmission. In color LCDs, each pixel includes sub-pixels equipped with color filters, respectively, such that light transmission through each sub-pixel is limited to the transmission band of the respective color filter. Most often, each pixel has three color filters configured to transmit red, green, and blue light, respectively, so as to provide realistic color imagery across the visible spectrum. The LCD is illuminated by a light source emitting essentially white light, or by a light source emitting light in several different bands within the visible spectrum, for example red, green, and blue light. However, LED-based illumination has gained popularity for several reasons, including reliability, low power consumption, low heat output, and wide color gamut (for color LCDs).
In one class of LED-backlit LCDs, an array of LEDs is placed behind the LCD, such that each LED illuminates only a local area of the LCD. Even though the light output from nearby LEDs may have some overlap on the LCD, this type of LED-backlit LCD facilitates a dual modulation scheme, in which the display image is achieved not only through adjusting the transmission of the LCD pixels but also through adjusting the light output of individual LEDs. For example, the light output of LEDs aimed at areas of the LCD displaying a dark portion of an image may be decreased. Such dual-modulation LED-backlit LCDs enables improved dynamic range and improved contrast, as well as reduced power output.
The light output of an LED is somewhat temperature sensitive. Generally, the output power decreases with temperature, and the LED temperature increases when the LED is driven harder to increase its power output. The temperature sensitivity may lead to distorted brightness and/or color representation in images displayed on an LED-backlit LCD as the temperature of the LEDs change during operation. This undesirable effect is of particular significance for dual-modulation LED-backlit LCDs, since the local temperature at any given LED changes as the drive value of the LED (or nearby LEDs) is changed.